documentclass{article}

usepackage{tikz}

% For use in nodestyle.tex
newlength{mnodesize}

begin{document}

% Default node styling.
begin{tikzpicture}
input{nodestyle}
node [inner] at (0, 0) {1};
end{tikzpicture}

% Smaller nodes (and text).
begin{tikzpicture}
newcommand{trnodesize}{1em}
% This currently has no effect:
newcommand{trnodefontsize}{tiny}
input{nodestyle}
node [inner] at (0, 0) {2};
end{tikzpicture}

end{document}

% Want a default value; most of the time 1.5em is ideal.
providecommand{trnodesize}{1.5em}
setlength{mnodesize}{trnodesize}
% Again, usually normalsize is fine.
providecommand{trnodefontsize}{normalsize}

tikzset{
  inner/.style = {
    align=center,
    inner sep=0pt,
    white,
    solid,
    fill=red,
    text centered,
    text width=mnodesize,
    minimum height=mnodesize,
    font=sffamily,
    % Doesn't work:
    % font=trnodefontsizesffamily,
  },
}
% So the next newcommand{trnodesize}{...} and
% newcommand{trnodefontsize}{...} will work.
lettrnodesizeundefined
lettrnodefontsizeundefined